Ocaml: How to define function with specific type

Question

I know this could be stupid question, but since I'm new to Ocaml, please give me some tips on defining functions with specific type.

I want to define function which type has int * int * (int -> int) -> int

So I made a function like this.


let rec sigma a b func:int->int= 
    if a >= b then func(a)
    else func(a) + sigma(a+1, b, func)

But sigma function here does not have type int * int * (int->int) -> int. Instead its type is int->int->(int->int)->int.

How should I define function sigma to have type int * int * (int->int) -> int?

(Error Message :


Error: This expression has type 'a * 'b * 'c
       but an expression was expected of type int

ivg ivg · Accepted Answer · 2019-09-12T15:34:06

The type int * int * (int->int) -> int denotes a function that takes a 3-tuple and returns and integer. A tuple (pair, triple, quadruple, etc) is written as (a,b,c,...), e.g.,

 let rec sigma (a,b,func) : int = 
    if a >= b then func(a)
    else func(a) + sigma(a+1, b, func)

With all that said, it is usually a bad idea to pass arguments via tuples in OCaml. It hampers currying, inefficient (each tuple is boxed into a separate value), and in general non-functional and not nice.

Also, when you constrain a parameter of a function, you shall parenthesize it, e.g., (func : int -> int).

And normally functions in OCaml are applied by juxtaposition of a function name and its arguments, e.g., given a function add

let add x y = x + y

we can apply (call it) just as add 3 5 (not add(3,5)!)

Therefore, the conventional way to define your sigma function would be

let rec sigma a b func : int = 
  if a >= b then func a
  else func a + sigma (a+1) b func

Ocaml: How to define function with specific type

1 Answers